We studied Cu intake and nutrition in exclusively breast-fed infants by determining Cu and ceruloplasmin concentrations in maternal and infant sera, as well as milk intakes and concentrations. The infants numbered 200 at birth, 116 at age 6 mo, 36 at 9 mo, and 7 at 12 mo. Postpartum the mean maternal serum concentrations of Cu and ceruloplasmin were high, but decreased in 4 mo to the level of nonpregnant women, and remained thereafter stable. The median milk Cu concentration decreased throughout lactation. In contrast, the mean infant serum concentrations of Cu and ceruloplasmin increased with age reaching adult levels by age 6 mo. The infant serum concentrations were independent of the milk concentrations which in turn were independent of the maternal serum concentrations and the degree of maternal supplementation (none, 2, or 4 mg Cu++ with Fe++ and Zn++). Neither maternal nor infant serum concentrations reflected intake of Cu. The daily Cu intake varied up to 10-fold between infants. No signs of Cu deficiency were detected during exclusive breast-feeding.